I’ve been doing it wrong. I was looking over creationist responses to my arguments that Haeckel’s embryos are being misused by the ID cretins, and I realized something: they don’t give a damn about Haeckel. They don’t know a thing about the history of embryology. They are utterly ignorant of modern developmental biology. Let me reduce it down for you, showing you the logic of science and creationism in the order they developed.
Here’s how the scientific and creationist thought about the embryological evidence evolves:
This is their problem: the similarities between embryos is an observed fact. It was recognized in the 18th century by observers like Goethe, reinforced by the work of the German naturphilosophen of the early 19th century, and far more persuasively, documented by the most eminent embryologist of that century, Karl Ernst von Baer, and even now, you can go to any college campus, even little ones like mine, and ask the local developmental biologist to show you his prepared slides of pig, chick, mouse, and fish embryos, and they will be able to show you all of the surprisingly consistent morphological features of these embryos. We can also show you photographs that reveal the breadth of the similarities, while also showing you the interesting little differences that have accumulated in different lineages.
The similarities between embryos are simply not matters to argue over. It’s settled. Has been for almost two centuries now.
Now if I were a creationist, I would have granted that, and instead gone on to criticize step 2, the hypothesis that these similarities are evidence of common ancestry. That’s what smart people did: Karl Ernst von Baer, for instance, was a creationist who dissented from Darwin’s interpretation, and he tried to explain the similarities as the product of the generality of early embryonic form, ascribing them to the relatively undifferentiated state of that stage. Von Baer’s explanation is actually a fairly good one, and I teach my students about it, and I even think it is still part of the explanation.
Step 3 is a problem for these alternative explanations, however. We keep getting more data, and the core processes of this developmental event show deeper and deeper similarities — and we can’t just wave it off as a result of general undifferentiation, because very specific shared genes are involved.
Unlike von Baer, modern day creationists tend not to be very smart people, and are uniformly uninformed about science. That’s the only way they can come up with the peculiarly strained logic I’ve illustrated above. They have a clear difference from evolutionary thought, in that they think first of all that their premise of a designer is true, but then what they do is let that lead them in irrational directions. I would have thought the battleground in this narrow slice of the debate would have been the second step, where evolutionists hypothesize common descent as an explanation of embryonic similarities — since they don’t accept common descent, they should be working hard to propose and test alternative hypotheses for the phenomenon.
I guess “working hard” is anathema to creationists, as is using their imaginations to come up with useful alternatives, because they don’t do that. Instead, they’ve taken the incredibly stupid tack of drawing that unnecessary third step, concluding that embryos don’t resemble one another, choosing that as their sticking point. It’s unnecessary, since if they’re claiming a single common designer, they could use that as an explanation for embryonic similarities, and it’s stupid, because as I said, the similarities are an observed fact.
Another example of this twisted logic can be seen in their carping about junk DNA. It’s a fact that most of the genome is useless, repetitive, or damaged junk, and we offer as an explanation that it is an accident of history, the accumulation of relics of past errors; creationists, instead of arguing with the interpretation and offering different explanations of their own, instead try to deny the evidence and pretend it is all functional. Maybe there is a method to their madness, though, because it also puts us a bit off balance — it’s hard to believe that they are actually making an argument that crazy.
Haeckel is just an unfortunate pawn in their game. He was guilty of fudging on one illustration…what the creationists don’t tell you is that it wasn’t the illustration that the textbooks have continued to use! His contemporaries as well as modern biologists don’t argue with the thrust of the diagram — that there is a stage at which vertebrate embryos resemble one another — and we also agree that Haeckel oversimplified and reduced differences for his pedagogical purposes. This is the diagram, of a dog, a chicken, and a turtle, that Haeckel was caught faking.
As you can see in the shared error in the somites that are highlighted in the box, this is the same woodcut used three different times. He was rightly told that this is an unethical shortcut to take, but note also: this is definitely not the diagram the textbooks have been using over and over.
Haeckel’s error was not in scientific step 1, the observations — it was that he overinterpreted (step 2), making grand pronouncements about the nature of embryonic and evolutionary change that were not out of line for his time, but have since been found to be invalid. His hypothesis went too far beyond explaining similarities as a result of history, to also claim a too-linear view of evolutionary change.
But, like I said, Haeckel doesn’t matter in this argument. He’s just a convenient whipping boy for the creationists, an excuse for them to try and deny the evidence. Step three, the ongoing confirmation of evolutionary explanations, is where the action is really at, and where their attempts to focus on 18th century speculation instead of 21st century science is simply a distraction.
This was brought home to me this morning as I was reading a paper by some zebrafish researchers on the development of the ventricles of the brain. This is a familiar topic to me — I’ve been studying zebrafish nervous system development since about 1980 — and my main interest was in finding out about these fluid-filled chambers in the brain, which I’ve usually ignored while I was focused on the neural epithelium.
The vertebrate brain is like a water balloon, hollow on the inside with a shell of cells around the outside. Initially, the cellular part is fairly thin, only one cell layer thick, but it obviously gets much thicker as development proceeds, to the point where in the adult the ventricles are actually fairly small (but important!) chambers buried deep in the mass of the brain.
The embryonic nervous system also partitions itself, making constrictions that block out the major domains of the forebrain, midbrain, and hindbrain. Again, I can’t help but compare this to a balloon: it’s like making a balloon animal, where the tube is subdivided into parts. Here’s a diagram of these early events:
What I thought was interesting was that I was reading this paper as a description of zebrafish brain development, and I glanced at the diagram and thought that it was a bit cartoony, but yes, that’s my familiar embryonic zebrafish brain…and then, of course, you look at the caption and it’s describing human brain development.
It wasn’t a surprise. I knew how human brains develop, too, and if I’d been reading about human ventricles, I would have given the diagram a nod of agreement. If it had been about mouse brain, the same; likewise, if it had said chick brain, I wouldn’t have given it a second thought. They are all roughly the same at this early stage of development. The same divisions, similar flexures, roughly the same kind of organization.
Of course there are differences, and the paper discusses those. In particular, fish nervous systems are initially solid instead of hollow, and secondarily form ventricular spaces after the subdivision of the brain into regions occurs; a mammalian brain starts hollow and stays hollow. There are also some molecular subtleties that haven’t been worked out in detail in multiple species, so some other differences may emerge as we learn more. And there certainly are differences that develop later — I would never mistake the adult human brain for the adult fish brain.
The similarities are real and go even deeper than what the 19th century biologists claimed. This paper also included a simple but interesting illustration comparing the organizations of the embryonic ventricles that resembles in principle the diagrams Haeckel drew — our human embryos had brains organized in the same way as those of fish embryos.
If the Discovery Institute had their way, that comparison would not be allowed to be made in our textbooks. They claim that they want better and more accurate science taught, but they really don’t: if we put more detail in the biology textbooks, it will only make the point of the similarities of vertebrate embryos stronger.
Lowery LA, Sive H (2009) Totally tubular: the mystery behind function and origin of the brain ventricular system. BioEssays 31:446-458.